NPE in ExpressionOperator.printCollection - backport from 2.7 (#2139)

Fixes #2136 
Backport from #2137
---------

Signed-off-by: Radek Felcman <radek.felcman@oracle.com>

Author: rfelcman

foundation/org.eclipse.persistence.core/src/main/java/org/eclipse/persistence/expressions/ExpressionOperator.java

@@ -2383,8 +2383,7 @@ public void printCollection(List<Expression> items, ExpressionSQLPrinter printer
 
         int[] indices = getArgumentIndices(items.size());
         String[] dbStrings = getDatabaseStrings(items.size());
-        for (int i = 0; i < indices.length; i++) {
-            final int index = indices[i];
+        for (final int index : indices) {
             Expression item = items.get(index);
 
             if ((this.selector == Ref) || ((this.selector == Deref) && (item.isObjectExpression()))) {